This invention relates to a device for measuring the distance between a pair of walls, and particularly to a device for measuring the radial distance between a pair of co-axial cylindrical walls which are spaced apart by an open annular area, or annulus. An example of a structure which includes such an annulus is a radial flow reactor of the moving bed type used in the petrochemical industry. Typically, such a reactor might include an assembly of two vertically positioned concentric cylindrical screens which are about 8-12 meters high and made of stacked tiers, each tier being about 2 meters high and formed of three or more curved panel segments which are welded together. The inner screen might have an outer diameter of about 1.8-2.4 meters and be separated from the outer screen by an annulus having a radial dimension of about 15-33 cm. In use, the annulus would be filled with catalyst pellets which are typically a ceramic substrate which is coated with a costly material, such as platinum. The catalyst pellets form a bed, through which a gas to be treated would typically pass in its path from an annular chamber outside the screen assembly to a hollow chamber within the inner screen. In order to ensure uniform treatment of the gas passing through the catalyst bed, it is essential that the annulus, and thus the catalyst bed, have a predetermined minimum thickness, and preferably be of a uniform thickness. If one region is of substantially less thickness than another, it will present less resistance to flow and thus result in less contact time between the gas and the catalyst in the particular region. It would also cause an overall increase in gas flow through the affected region which could possibly cause "hot spots" to be created in the bed and a deterioration of the catalyst if the reactor is operated at its highest capacity. Alternatively, to prevent catalyst deterioration and "hot spots", a reactor having a non-uniform annulus could be operated at a lower than normal gas flow rate but this would reduce its production capacity. Furthermore, since the catalyst is extremely expensive, it is highly desirable that none be wasted by having some portions of the annulus wider than other portions. In order to ensure that the annulus is of the proper minimum and maximum dimension, it is essential that it be measured prior to the installation of catalyst. Measurements are typically required to be taken at about every 30 cm along the length of the screens and at every 45.degree. around their circumference, as well as at the top and bottom of each tier and along every vertical weld seam between segments. Thus, about 500 or so measurements must usually be taken for a screen assembly having a length of about 12 meters. This operation is very tedious, and typically requires at least two workers to each expend 6-8 hours of time. It also necessitates either the erection of scaffolding around the screen assemblies or, more often, the lowering of a worker, suspended by a harness assembly, into the center screen opening. The worker doing the measuring uses a thin metal scale or ruler which he must carefully pass through a slot in the inner screen and, while trying to keep the scale level and aimed perpendicular to the screen, push it into the slot until its end engages the inner surface of the outer screen. At this point, he must take a reading of the distance to the outside screen surface. The series of readings must be carefully noted and a correction factor must later be applied to subtract the thickness of the inner screen wire. Because of the nature of the procedure, it is quite possible for repetitive readings taken by the same or different persons to vary as much as 2 or 3 min.
The measuring operation is usually performed at least several times. Initially when the screen segments are made, when they are fabricated into long sections, and after installation at the job site. If the readings show that the annulus varies from a very tight tolerance, adjustments might be able to be made be changing the angular position of the inside screen relative to the outside screen. More commonly, however, hydraulic jacks are used to selectively adjust the internal diameter of the screen at one or more locations. Obviously, every time such an adjustment is made a new set of readings must be made.